A conventional heat pump type drying apparatus uses a heat pump as a heat source, and circulates drying air. For example, FIG. 6 shows the conventional heat pump type drying apparatus as described in Japanese Patent Application Laid-open No.H7-178289.
In FIG. 6, a clothes drying apparatus body 1 includes a rotation drum 2 used as a dry room which is rotatably provided in the body 1. The body 1 is operated by a motor 3 through a drum belt 4. A blower 22 sends drying air from the rotation drum 2 to a circulation duct 18 through a filter 11 and a rotation drum-side air intake 10. The blower 22 is operated by the motor 3 through a fan belt 8. A heat pump apparatus comprises an evaporator 23 which evaporates a refrigerant to dehumidify the drying air, a condenser 24 which condenses the refrigerant and heats the drying air, a compressor 25 for generating a pressure difference in the refrigerant, an expansion mechanism 26 such as a capillary tube for maintaining the pressure difference of the refrigerant, and a pipe 27 through which the refrigerant flows. A portion of drying air heated by the condenser 24 is discharged out from the body 1 through an exhaust port 28. The arrow B shows a flow of the drying air.
Next, an operation of the above mentioned conventional apparatus will be explained. First, clothes 21 to be dried are put in the rotation drum 2. Next, if the motor 3 is operated, the rotation drum 2 and the blower 22 are rotated and a flow B of the drying air is generated. The drying air deprives moisture from the clothes 21 in the rotation drum 2, and as a result, the drying air which includes the moisture is sent into the evaporator 23 of the heat pump apparatus through the circulation duct 18 by the blower 22. The drying air from which heat is deprived by the evaporator 23 is dehumidified, the drying air is sent to the condenser 24 and is heated, and then the drying air is circulated back into the rotation drum 2. An exhaust port 19 is provided in an intermediate portion of the circulation duct 18. Drain water which has been generated and dehumidified by the evaporator 23 is discharged through the exhaust port 19. As a result, the clothes 21 are dried.
However, the above conventional structure has a problem that the compressor compresses liquid when the heat pump is operated in a high temperature atmosphere and under a low air quantity condition.
A situation in which the compressor compresses liquid when the heat pump is operated in the high temperature atmosphere will be explained. In the heat pump type drying apparatus having the circulation duct, an input from an external power supply to the compressor and an amount of heat released outside from air circulating in the duct become equal to each other. That is, if the input to the compressor is constant, a difference between the atmosphere temperature and the average temperature of air in the circulation duct is always constant. Therefore, if the atmosphere temperature rises, the average temperature of air in the circulation duct also rises. For this reason, if the refrigerant pressure sucked by the compressor rises, the discharged refrigerant pressure also rises, and the pressure exceeds a permissible pressure of the compressor. As a measure against this problem, in a current product, an input (frequency) of the compressor is lowered in the high temperature atmosphere. By using this measure, the average temperature of the air in the duct is lowered, and the permissible pressure of the compressor is maintained. However, there are problems that the frequency in the compressor is reduced, the refrigerant circulation amount is also reduced and thus, a heat exchange amount of the evaporator is reduced, and the refrigerant is not completely vaporized in the evaporator. Liquid refrigerant remaining in the evaporator exit becomes a cause of liquid compression of the compressor. If the compressor compresses liquid, a stress exceeding the permissible value is applied to the compressor, and there is an adverse possibility that the constituent part is damaged.
Next, a situation in which the compressor compresses liquid when the heat pump is operated under the low air quantity condition will be explained. If the air quantity is reduced, the air-side heat transfer coefficient in a radiator and the evaporator is lowered. Therefore, a temperature difference between the refrigerant and air which is required for securing the same heat exchange amount is increased, a compressor sucking pressure is lowered and a discharging pressure is increased. In this case, the input (frequency) of the compressor is also controlled to be reduced so as to maintain the permissible pressure of the compressor similar to the case in which the compressor is operated in the high temperature atmosphere. As a result, there is a problem that the refrigerant is not completely vaporized in the evaporator.
Further, in the conventional structure, there is a problem that when the heat pump is actuated in a low temperature atmosphere or under the low air quantity condition, since the evaporator pressure, i.e., the evaporator temperature is lowered, frost forms on the evaporator.
Next, a condition in which the evaporator pressure is lowered when the heat pump is operated in the low temperature atmosphere will be explained. As described above, if the inputs to the compressor are the same, a difference between the atmosphere temperature and the average temperature of air in the circulation duct is always constant. Therefore, if the atmosphere temperature is lowered, the average temperature of air in the circulation duct is lowered. For this reason, a pressure of a refrigerant which is discharged from and sucked by the compressor is lowered, and the temperature of the refrigerant in the evaporator becomes lower than 0°, and there is a problem that frost forms on the evaporator.
Next, a condition in which a pressure in the evaporator is lowered when the heat pump is operated under the low air quantity condition will be explained. As described above, if the air quantity is reduced, the compressor sucking pressure is lowered, and the discharging pressure is increased. If the sucking pressure is lowered, the temperature of the refrigerant in the evaporator becomes lower than 0 and there is a problem that frost forms on the evaporator.
Further, HFC refrigerant (including atoms of hydrogen, fluorine and carbon in its molecule) which is currently used as a refrigerant of the heat pump apparatus directly affects the global warming. Thus, a natural refrigerant such as CO2 existing in the nature has been proposed as an alternative refrigerant. However, if CO2 refrigerant is used, a theoretical efficiency of the heat pump system is lower than that of the HFC refrigerant, and there is a problem that the operating efficiency of the heat pump type drying apparatus is lowered.
Therefore, using a natural refrigerant such as CO2 which does not directly affect the global warming, it is necessary to realize energy-conservation and a high efficiency so as to reduce the indirect influence on the global warming.
The present invention has been accomplished to solve the conventional problems, and it is an object of the invention to provide a heat pump type drying apparatus in which when a refrigerant which can be brought into a supercritical state on the radiating side of a heat pump cycle such as CO2 is used, compression of liquid refrigerant of the compressor and pressure reduction of the evaporator can be avoided even in a high/low temperature atmosphere and under the low air quantity condition, and high efficiency is realized.